1. Field of the Invention
The present invention relates to an optical recording medium irradiated with light to record or reproduce information. Particularly, the present invention relates to a layer-interval structure of an optical recording medium having three or more information recording surfaces, and relates to a method or a device for reproducing information on the multilayer optical recording medium or recording information therein.
2. Description of the Background Art
A high-density and large-capacity optical information recording medium is currently known, for example, as an optical disk such as a DVD or a BD. The optical disk has recently become increasingly popular as a recording medium for recording an image, music or computer data. In order to increase the recording capacity, an optical recording medium having a plurality of recording layers has been offered, as described in Japanese Patent Laid-Open Publication No. 2001-155380 and/or Japanese Patent Laid-Open Publication No. 2008-117513.
FIG. 13 shows a conventional configuration of an optical recording medium and an optical pickup. A divergent beam 70 emitted from alight source 1: transmits a collimating lens 53 provided with a spherical-aberration correcting means 93; is incident upon a polarization beam splitter 52 and transmits it; transmits a quarter-wave plate 54 to convert into a circularly polarized beam; thereafter, is converted into a convergent beam by an objective lens 56; and transmits a transparent substrate of an optical recording medium 401 to concentrate upon any of recording surfaces 401a, 401b, 401c and 401d formed inside of the optical recording medium 401. The objective lens 56 is designed in such a way that the spherical aberration is zero in the middle depth position between the first recording surface 401a and the fourth recording surface 401d. The spherical-aberration correcting means 93 moves the collimating lens 53 in the optical-axis directions to thereby remove a spherical aberration generated when a beam converges upon each recording surface 401a, 401b, 401c, 401d. 
The objective lens 56 is provided with an aperture portion 55 restricting the aperture thereof and has a numerical aperture NA of 0.85. The beam 70 reflected by the fourth recording surface 401d transmits the objective lens 56 and the quarter-wave plate 54 to convert into a linearly polarized beam different by an angle of 90 degrees from the outward path. Thereafter, the beam 70 is reflected by the polarization beam splitter 52; a condensing lens 59 is utilized to convert into a convergent beam; and the convergent beam is given an astigmatism through a cylindrical lens 57, so as to be incident upon a photodetector 320.
The photodetector 320 includes four light-receiving portions (not shown) each outputting an electric-current signal according to the quantity of received light. Each electric-current signal is used for generating a focus error (FE) signal in an astigmatism method, a tracking error (TE) signal in a push-pull method and an information (RF) signal recorded in the optical recording medium 401. The FE signal and the TE signal are amplified to a desired level and compensated for phase, and thereafter, are supplied to actuators 91 and 92 for focus and tracking control.
Herein, a problem arises if thicknesses t1 to t4 are all equal, as follows. For example, in order to execute recording and reproduction for the fourth recording surface 401d, the beam 70 is concentrated on there, and then, a part of the beam 70 is reflected by the third recording surface 401c. Since the distance between the third recording surface 401c and the fourth recording surface 401d is equal to the distance between the third recording surface 401c and the second recording surface 401b, the part of the beam 70 reflected by the third recording surface 401c forms an image on the back side of the second recording surface 401b, and the reflected beam by the second recording surface 401b is reflected again by the third recording surface 401c and gets mixed with a reflected beam from the fourth recording surface 401d which should be naturally read. Further, since the distance between the second recording surface 401b and the fourth recording surface 401d is also equal to the distance between the second recording surface 401b and a face 401z of the optical recording medium 401, a part of the beam 70 reflected by the second recording surface 401b forms an image on the back side of the face 401z of the optical recording medium 401, and the reflected beam by the face 401z is reflected again by the second recording surface 401b and gets mixed with the reflected beam from the fourth recording surface 401d which should be naturally read. This causes the problem of superimposing multiple reflected beams from images formed on the backsides of other layers on the reflected beam from the fourth recording surface 401d which should be naturally read to thereby hinder the recording/reproduction. The mixed beams tend to interfere and form a brightness distribution through interference on a light-receiving element, and the brightness distribution varies according to the change in the phase difference between the reflected beam from the fourth recording surface 401d and a reflected beam from another layer which is caused by a slight dispersion of intermediate-layer thicknesses inside of the face of an optical disk, thereby significantly deteriorating the quality of a servo signal and a reproductive signal. Below, this problem is referred to as a back-focus problem.
In order to prevent this back-focus problem, a method is disclosed of gradually lengthening the distance between each recording layer one by one from the face 401z of the optical recording medium 401 in such a way that no image is formed on the back side of the second recording surface 401b or the back side of the face 401z at the same time that the beam 70 is concentrated on the fourth recording surface 401d from which reading should be naturally executed (refer to Japanese Patent Laid-Open Publication No. 2001-155380). Herein, the thicknesses t1 to t4 each have a manufacturing dispersion of ±10 μm, and even if they are widely dispersed, each thickness t1 to t4 needs to have a different distance, thereby setting the difference in distance, for example, to 20 μm. In this case, t1=40 μm, t2=60 μm, t3=80 μm and t4=100 μm, then a total thickness t (=t2+t3+t4) from the first recording surface 401a to the fourth recording surface 401d becomes 240 μm.
If the thickness of a cover layer between the face and the first recording surface 401a is equal to the thickness between the fourth recording surface 401d and the first recording surface 401a, then a beam reflected by the fourth recording surface 401d is focused at the face and reflected from there, is reflected again by the fourth recording surface 401d, and thereafter, is led to the light-receiving portions. Because of the back focus at the face, this luminous flux does not have information such as a pit and a mark contained in a back-focus luminous flux on another recording layer. However, if there are a large number of recording layers, the quantity of light returning from the recording layers decreases to thereby heighten the reflectance of the face relatively. Accordingly, interference with a luminous flux on a reproduction layer occurs likewise, thereby significantly deteriorating the quality of a servo signal and a reproductive signal.
Taking the above problems into account, Japanese Patent Laid-Open Publication No. 2008-117513 suggests the distance between recording layers in an optical disk and discloses a structure as follows.
An optical recording medium includes four information recording surfaces as first to fourth information recording surfaces in order from the face of the optical recording medium. The distance between the face and the first information recording surface is 47 μm or below, and the intermediate-layer thickness between each information surface from the first information recording surface to the fourth information recording surface is a combination of 11-15 μm, 16-21 μm, and 22 μm or above. The distance between the face and the fourth information recording surface is 100 μm.
The distance between the face and the first information recording surface is 47 μm or below and the distance between the face and the fourth information recording surface is 100 μm.
In an optical disk system, a beam of light is incident upon the face of an optical disk and reflected by a recording surface, and the reflected beam is detected. Hence, an influence is also given by the refractive index of a transparent material transmitting the beam from the face to an optical-disk surface. In the disk structures of Japanese Patent Laid-Open Publication No. 2001-155380 and Japanese Patent Laid-Open Publication No. 2008-117513, however, neither an examination nor a description is given about the refractive index of a transparent material, and thus, an effect given by the refractive index is not considered at all.